With recent advancement of digital technologies, electronic hardware such as portable information devices and home information appliances have been developed to provide higher functionality. For this reason, there have been increasing demands for an increase in a capacity of nonvolatile memory elements, reduction in a write electric power in the memory elements, reduction in write/read time in the memory elements, and longer life of the memory elements.
Under the circumstances in which there are such demands, it is said that there is a limitation on miniaturization of the existing flash memory using a floating gate. Accordingly, in recent years, a novel resistance variable nonvolatile memory element using a resistance variable layer as a material of a memory section has attracted an attention.
The resistance variable nonvolatile memory element basically has a very simple structure in which a resistance variable layer is sandwiched between a lower electrode and an upper electrode. Upon application of a predetermined electric pulse having a voltage of a magnitude which is not smaller than a threshold between the upper and lower electrodes, the nonvolatile memory element switches to a high-resistance state or to a low-resistance state. By corresponding these different resistance states to numeric values, respectively, data is stored. Because of such a simple structure and operation, it is expected that the resistance variable nonvolatile memory element has a potential to achieve further miniaturization and cost reduction. Furthermore, Since switching to the high-resistance state or to the low-resistance state sometimes occurs in an order of 100 ns or less, the resistance variable nonvolatile memory element has attracted an attention for achievement of a high-speed operation, and a variety of proposals have been proposed.
For example, Patent document 1 discloses a resistance variable nonvolatile memory element in which metal ions are caused to travel into and out of a resistance variable layer 3302 to produce a high-resistance state and a low-resistance state by applying voltages between an upper electrode and a lower electrode, thereby storing data. Also, as disclosed in Patent document 2, there is known a resistance variable memory in which crystalline states of a resistance variable layer are changed using electric pulses to switch its resistance states.
In addition to the above, there are numerous proposals for resistance variable nonvolatile memory elements using metal oxides for the resistance variable layer 3302.
The resistance variable nonvolatile memory elements using the metal oxides are classified into two major kinds depending on the material used for the resistance variable layer. Patent document 3 or the like disclose one kind of resistance variable nonvolatile memory element using perovskite materials (Pr (1-x) CaXMnO3 (PCMO), LaSrMnO3 (LSMO), GdBaCoxOy (GBCO), etc.) as the resistance variable layer.
The other kind is resistance variable nonvolatile memory elements using binary transition metal oxides. Since the binary transition metal oxides have a very simple composition as compared to the above illustrated perovskite materials, composition control and layer deposition in manufacturing are relatively easy. In addition, the binary transition metal oxides have an advantage that they are relatively highly compatible with a semiconductor manufacturing process. For these reasons, the binary transition metal oxides have been recently vigorously developed. For example, Patent document 4 discloses NiO, V2O5, ZnO, Nb2O5, TiO2, WO3, and CoO as the resistance variable materials. Patent document 5 or Non-patent documents 1 to 3 disclose resistance variable memory elements using transition metal oxides such as Ni, Nb, Ti, Zr, Hf, Co, Fe, Cu, Cr, etc., especially resistance variable element using as resistance variable materials, oxides which are oxygen-deficient in stoichiometric composition (hereinafter referred to as oxygen-deficient oxides).
The oxygen-deficient oxide will be described in more detail. For example, in the case of Ni, NiO is known as an oxide having a stoichiometric composition. NiO contains 0 atoms and Ni atoms which are equal in number and is represented by 50 at % in terms of oxygen content. The oxide having lower oxygen content than the oxygen content 50 at % is called an oxygen-deficient oxide. In this example, the oxide is a Ni oxide and therefore may be expressed as an oxygen-deficient Ni oxide.
Patent document 6 or Non-patent document 2 disclose examples in which a structure formed by oxidating a surface of titanium nitride to form a crystalline titanium oxide (TiO2) film of a nanometer order is used as the resistance variable layer.
Regarding how the resistance changing phenomenon occurs, the nonvolatile memory elements using the above metal oxides are categorized into two kinds. One kind is a unipolar nonvolatile memory element which changes its resistance in response to electric pulses which are the same in polarity and different in voltage magnitude (e.g., the resistance value is increased or decreased by applying voltages of +1V and +2V). The nonvolatile memory elements disclosed in patent document 4 or 5 are the unipolar nonvolatile elements. The other kind is a bipolar nonvolatile memory element which is controlled to change its resistance in response to electric pulses having voltages with different polarities (e.g., the resistance value is increased or decreased by applying voltages of +1V and −1V). The nonvolatile memory element which performs such resistance changing phenomenon is disclosed in patent document 3 or 6.
Patent document 5 discloses, as the materials of the upper and lower electrodes sandwiching the resistance variable layer, for example, iridium (Ir), platinum (Pt), ruthenium (Ru), tungsten (W), oxides of Ir and Ru, nitride of titanium (Ti), polysilicon, etc. Patent document 6 discloses nonvolatile memory elements using as electrode materials, Pt, Ir, osmium (Os), Ru, rhodium (Rh), palladium (Pd), Ti, cobalt (Co), W, etc.
Patent document 7 discloses nickel (Ni), silver (Ag), gold (Au), Pt. Patent document 8 discloses Pt, Ir, Ru, Ir oxide and Ru oxide.
Patent document 1: Japanese Laid-Open Patent Application Publication No. 2006-40946
Patent document 2: Japanese Laid-Open Patent Application Publication No. 2004-349689
Patent document 3: U.S. Pat. No. 6,473,332
Patent document 4: Japanese Laid-Open Patent Application Publication No. 2004-363604
Patent document 5: Japanese Laid-Open Patent Application Publication No. 2005-317976
Patent document 6: Japanese Laid-Open Patent Application Publication No. 2007-180202
Patent document 7: Japanese Laid-Open Patent Application Publication No. 2007-88349
Patent document 8: Japanese Laid-Open Patent Application Publication No. 2006-324447
Non-patent document 1: I. G. Beak et al., Tech. Digest IEDM 2004, page 587
Non-patent document 2: M, Fujimoto et al., Japanese Journal of Applied Physics Vol. 45 2006, L310-L312 page
Non-patent document 3: A. Chen et al. Tech. Digest IEDM 2005, page 746